The present invention relates to a driving mechanism for a photographic lens adapted to move the photographic lens back and forth in parallel to an optical axis of the photographic lens for power variation or focusing.
As photographic cameras become more popular, marketing trends require that cameras should be more and more compact, thin, and lightweight and simultaneously contain a power varying mechanism such as a zoom mechanism or focus change-over mechanism. The power varying mechanism functions to move the photographic lens back and forth in a direction parallel to an optical axis, for example, to move the photographic lens outward with respect to a camera body for telephoto, and move the photographic lens back toward the camera body for wide-angle or standard photo. Thus, a lens barrel holding the photographic lens is moved back and forth relative to the camera body in parallel to the optical axis.
In order to assure that the photographic lens can be moved within a given range to obtain a desired magnification while making the camera as thin as possible, the lens barrel is typically constructed with four stages of which three stages can be successively moved. More specifically, the lens barrel should be constructed so that a first driving lens barrel is movable back and forth relative to a stationary lens barrel, a second driving lens barrel is movable back and forth relative to the first driving lens barrel, and a movable lens barrel holding a part of the photographic lens is movable back and forth relative to the second driving lens barrel. By adopting such an arrangement, it is possible to limit a length of the lens barrel in the direction of the optical axis approximate to a length of the stationary lens barrel and to obtain a moving range of the photographic lens approximately corresponding to three times the length of the stationary lens barrel. By adopting such arrangement, it is possible to limit a length of the lens barrel in the direction of the optical axis relative to a length of the stationary lens barrel and to obtain a moving range of the photographic lens approximately corresponding to three times the length of the stationary lens barrel. The respective stages of the lens barrel must keep the photographic optical system in a predetermined optical relationship during their movement.
A driving mechanism to move the lens barrel back and forth utilizing a cam barrel is also well-known and the above-mentioned arrangement can be adopted in such a driving mechanism. By utilizing a single cam, a single power source may be used and cost savings may be realized. However, a length of the cam will be unacceptably increased to assume an adequate moving range of the lens barrel while making the camera body as thin as possible. Consequently, a cam slit must be formed in the lens barrel over its full length, resulting in an apprehension that the strength of the lens barrel might be unacceptably decreased. Particularly, in the case of the photographic lens adopting a power varying/focusing mechanism which relies upon a single cam, the cam will be inevitably made longer and the cam strength will correspondingly decrease. To avoid such an inconvenience, a cam groove having a bottom, instead of a cam in the form of a through-hole, has been adopted as the cam formed in the lens barrel.
It is also well-known from Japanese Patent Application Disclosure Gazette No. 1995-128567 to move one driving lens barrel back and forth in parallel to the optical axis without rotation and to rotate the other lens barrel about the optical axis to thereby move it back and forth in parallel to the optical axis so that a front lens group frame may be moved back and forth in parallel to the optical axis as the other lens barrel rotates.
However, the driving mechanism formed with the previously-described cam groove having the bottom suffers from the following problem. With the conventional mechanism utilizing the cam slit formed in the lens barrel to drive the photographic lens, a pin member interlocked with the cam slit is operatively associated with the cam slits of two lens barrel stages, one serving to guide a movement in parallel to the optical axis and the other serving to regulate a distance of this movement. With the driving mechanism adopting the cam in the form of a groove having a bottom, on the other hand, it is impossible for the pin member to extend through the lens barrel and therefore, in addition to the lens barrel state formed with the cam groove, a driving ring member or the like must be provided to control a direction as well as a distance of the pin member's movement. In the case of a triple stage zoom lens barrel described in Japanese Patent Application Disclosure Gazette No. 1995-128567, a stationary lens barrel comprises inner and outer barrels between which a rotatable barrel is disposed. Such double barrel arrangement correspondingly enlarges the outer diameter of the lens barrel and limits the possibility of making the camera compact.